The present invention relates to a method and an arrangement in connection with smelt removal from a recovery boiler, whereby the smelt is removed from the boiler via a smelt spout surrounded outside the boiler by a hood, and a dispersion medium, preferably steam, is sprayed via a nozzle into the smelt flow falling from the smelt spout for shattering the smelt flow. Specifically, the invention relates to a washing arrangement for washing the hood and the feed device for the dispersion medium.
An essential apparatus in the recovery cycles of sulfate and other Na-based pulping processes is the recovery boiler for waste liquor containing cooking chemicals, such as a soda recovery boiler, wherein the chemicals are processed into a form suitable for recovery purposes. In a sulfate process, the most important chemicals are sodium and sulfur. Organic substances dissolved during the digestion in the waste liquor are combusted in the furnace of the boiler generating heat, which is utilized on one hand for converting inorganic compounds of the waste liquor back into chemicals to be used in cooking and on the other hand for generating steam. The inorganic substance in the waste liquor melts in the high temperature of the furnace and flows as smelt onto the bottom of the furnace.
From the bottom of the boiler the chemical smelt is led via cooled smelt spouts into a tank, where it is dissolved in water or weak white liquor for forming soda lye, i.e. green liquor. In the sulfate process, the main components of smelt and thus green liquor are sodium sulfide and sodium carbonate. The green liquor is then led to a causticizing plant, where white liquor is produced therefrom.
The hot smelt flow causes crashes or explosions when falling into a dissolver tank. The noise is due to explosion reactions between the smelt and water as the smelt gets into contact with the green liquor in the dissolver tank. The temperature of the smelt is in the order of 750-820° C., and the temperature of the green liquor (or weak white liquor), containing mainly water, in the dissolver tank is in the order of 70-100° C.
The intensity of the explosion reactions taking place in the dissolving tank may be regulated by shattering the smelt flow exiting the smelt spout into small parts before it gets into contact with the green liquor in the dissolving tank.
Smelt shattering is most often done by directing a steam jet and/or green liquor jet against the smelt flow exiting the smelt spout. Also a jet formed of mist generated from air and water has been suggested. The most common smelt shattering method practiced in Finland is the use of low or medium pressure steam.
The part of the smelt spout extending outside the furnace wall is usually surrounded by a closed hood, i.e. protective housing, which prevents liquid and smelt splashes and vent vapors from entering the surroundings. The bottom part of the hood is in connection with a smelt dissolving tank located underneath the smelt spout, which tank receives the smelt from the spout and in which tank the smelt is dissolved in liquid forming green liquor. The nozzles spraying the medium dispersing the smelt flow are typically installed in the hood and directed towards the smelt flow falling from the spout. Smelt splashes may enter and stick in the hood and on the walls of the dissolving tank. Smelt cakes thus formed cause explosions when falling in the dissolving tank. Thus, the hood is subject to hot and corroding conditions caused by the smelt. Therefore, the interior of the hood has been washed, typically with weak liquor. The circumference of the hood may be provided with wash distributor pipe for washing off splash smelt from the walls of the hood and for preventing smelt deposits.
Splash smelt can also enter a nozzle spraying a smelt flow dispersing medium, such as steam, and thus shorten the operating life of the nozzle.